1. Field of the Invention
The present invention relates to an optical amplification device; and in particular, to an optical amplification apparatus for amplifying a plurality of light signals having different multiplexed wavelengths, and to a wavelength multiplexed light transfer apparatus for using this optical amplification device.
2. Related Arts
A system that has recently become popular provides for the increasing of the capacity of transfer data by employing multiplexing wavelengths of light signals having different wavelengths and inputting them to a signal transfer path. The light transfer system, which has a large capacity, requires the inclusion of a direct optical amplification device, such as an optical fiber amplification device, in order to reduce the costs of a relay device.
For an optical fiber amplification device that is currently employed, a light divider is provided at the output portion of an optical fiber amplifier in order to maintain a constant light output, and the light output is so controlled that a constant light input level is maintained at a light receiver connected to the light divider. Such a structure is well known wherein, when a plurality of light signals having different wavelengths are wavelength multiplexed signals, a dummy light is superimposed on the multiplexed light signals in addition to the wavelengths of their light signals, and a constant output by an optical amplifier is maintained by using a dummy light power level at the output portion of the optical amplifier. This structure is described in Japanese Patent Application No. Hei 4-78827, for example.
When a conventional optical fiber amplification device is employed for a wavelength multiplexed light communication device, a constant sum for the output levels of wavelength multiplexed light signals is maintained. Thus, if the number of multiplexed light wavelengths to be transferred is increased or reduced, due to the provision of more light transmitters or due to the abnormal functioning of a light transmitter, a signal light level for one wavelength of a light signal to be transferred is reduced or increased. A reduction or an increase in the signal level of one wavelength will result in the deterioration of the light signal transfer operation.
The wavelength multiplexed light transfer device, therefore, must maintain a constant transfer output level for light signals having different wavelengths. However, since the conventional optical fiber amplification device maintains a constant total for the light output power, when the number of multiplexed wavelengths is changed, the transfer output levels for the wavelengths are altered and degradation of the transfer characteristic occurs.
With the above structure wherein a dummy light is superimposed.on the light signal, the manufacturing costs are increased because a dummy light generator is required for a transmitter. In addition, when the dummy light generator malfunctions, the operation of the optical amplification device becomes unstable.